Bipolar transistors are electronic elements having three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. If the emitter and collector are formed using n-type dopants (n-doped) and the base is formed using p-type dopants (p-doped), then the element is an “npn” bipolar transistor. Conversely, if the emitter and collector are n-doped and the base is p-doped, the element is an “npn” bipolar transistor. Because electron mobility in the base region of npn transistors is higher than that of holes in the base of pnp transistors, npn transistors exhibit higher-frequency operation and higher-speed performances than pnp transistors.
A heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) in which the emitter and base regions are formed using crystalline semiconductor materials having different band gaps, whereby the interface between the different materials creates a heterojunction. References to SiGe HBTs herein are to npn-type HBTs in which the heterojunction is typically generated by forming a highly n-doped Si emitter structure on an SiGe base structure, which in turn is formed on an n-doped silicon substrate-based collector. SiGe HBTs may also be fabricated as pnp-type HBTs as well. SiGe HBTs are commonly used in modern communication circuits (e.g., radio-frequency (RF) power amplifiers used in cellular phones) because of their ability to handle high frequencies (i.e., up to several hundred GHz) with relatively low power consumption (i.e., in comparison to CMOS switches configured to achieve the same level of performance).
As semiconductor devices continue to decrease in size, optimal device operation becomes more difficult to achieve. For example, when scaling advanced SiGe HBTs two factors are of primary importance: 1) scaling the vertical emitter-base-collector dimension to improve cutoff frequency (Ft) and DC current gain (β), and 2) scaling the lateral dimension of the bipolar transistor to improve the maximum frequency of unity power gain (Fmax) and the RF noise figure (Nfmin). In SiGe HBTs, lateral scaling has typically focused on reducing the parasitic extrinsic base resistance (Rbx) and extrinsic base-collector capacitance (CBC).
State-of-the-art SiGe HBTs are configured using raised base techniques, which were not used in earlier HBTs. The raised base techniques include a raised extrinsic base region that serves to improve the fundamental tradeoff between base resistance and collector-base capacitance by separating the extrinsic base region from the intrinsic base growth. In a typical raised base scheme the raised extrinsic base material is Si, and is typically implemented using polycrystalline Si (polysilicon). However, conventional raised-base techniques utilize processes requiring separate deposition steps for creating an epitaxially grown extrinsic base region that connects the single-crystal intrinsic base region to the base contact. Consequently, fabrication of a silicon raised base often significantly increases manufacturing costs, and requires high temperature processing, and is accompanied by difficulty in etching and forming the silicon raised base due to low etchant selectivity during fabrication.
What is needed is a raised-based SiGe HBT having the enhanced performance characteristics provided by single-crystal extrinsic base structures, but can be fabricated in a way that avoids the difficulties encountered by conventional approaches.